Hach-Lange HACH 9523 - Manual (Page 8)

• Measurement in pure and ultrapure water, power plants, semiconductor industry, pharmaceutical
• Drinking water
• Industrial processes (chemistry, paper mills, sugar refineries, etc.)
Principle of operation (pH calculation)
The Polymetron 9523 analyzer adheres to the recommendations contained in the guidelines for
feedwaters, boiler water and steam quality for power and industrial plants.
The pH calculations can only be applied under the following strict chemical conditions:
• The sample must only contain an alkaline agent (ammonia, sodium hydroxide or ethanolamine)
• Any impurity is principally NaCl (sodium chloride)
• The concentration of impurity must be negligible in comparison to the alkaline agent
Control of conductivity values
Control of the minimum and maximum conductivity values depends upon the volatile conditioner.
These values are illustrated in the following two tables.
Note: AVT (All Volatile Treatment) - Conditioning concept where only volatile alkalizing agents are used; mainly
ammonia.
NH
3
Conditioner:
AVT
pH 7 - 10.0
Conductivity C1 (μS/cm) 2.8 - 28
Acid conductivity C2 (μS/cm) < 0.5
NaOH Conditioner:
AVT
pH 7 - 10.7
Conductivity C1 (μS/cm) 2.5 - 125
Acid conductivity C2 (μS/cm) < 100
Measurement of pH by differential conductivity
Measurement of pH in environments of low conductivity using the standard potentiometric method
(glass electrode + reference) is extremely delicate and not very accurate because it is proportional to
the concentration logarithm. It also requires a more frequent calibration to compensate for variations
in the measurement chain (junction potential, degradation of the glass membrane).
On the other hand, measurement of conductivity in these environments is a lot more reliable and
more accurate as it is directly proportional to the concentration in impurity, and in addition requires
little or no maintenance. Therefore, given the relationship between the pH and conductivity of a
product, the conductivity measurement can be used to determine a precise pH.
If the product contains impurities (generally in the form of salts), this calculation cannot be applied.
The principle is then to transform the salt into acid by passing it through a cationic resin and, given
the relationship of the conductivity between the acid and the corresponding salt (always around 3), to
determine the conductivity originating only from the conditioner:
ΔC = Conductivity before resin (C1) – Conductivity after resin (C2) / A
and
ph = f(ΔC)
Note: The calculated pH is the pH of the sample at the analyzer inlet (channel 1). The analyzer does not calculate
the pH downstream of the resin cartridge.
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